Method and system for providing a power lateral PNP transistor using a buried power buss

ABSTRACT

A power lateral PNP device is disclosed which includes an epitaxial layer; a first and second collector region embedded in the epitaxial layer; an emitter region between the first and second collector regions. Therefore slots are placed in each of the regions. Accordingly, in a first approach the standard process flow will be followed until reaching the point where contact openings and metal are to be processed. In this approach slots are etched that are preferably 5 to 6 um deep and 5 to 6 um wide. These slots are then oxidized and will be subsequently metalized. When used for making metal contacts to the buried layer or for ground the oxide is removed from the bottom of the slots by an anisotropic etch. Subsequently when these slots receive metal they will provide contacts to the buried layer where this is desired and to the substrate when a ground is desired. In a second approach the above-identified process is completed up through the slot process without processing the lateral PNPs. With a separate masking and etching the oxide is removed from the PNP slots and boron is deposited in a diffusion furnace and driven in a non oxidizing atmosphere.

CROSS-RELATED APLICATION

The present application is a divisional of application Ser. No.10/389,551, entitled “Method and System for Providing a Power LateralPNP Transistor Using a Buried power Buss”, filed Mar. 13, 2003; which isa divisional of application Ser. No. 10/176,285 entitled “Method andSystem for Providing a Power Lateral PNP Transistor Using a Buried PowerBuss”, filed Jun. 19, 2002, now U.S. Pat. No. 6,566,733.

FIELD OF THE INVENTION

The present invention relates specifically to high current semiconductordevices and more particularly to a power lateral PNP device using aburied power buss.

BACKGROUND OF THE INVENTION

Lateral PNP transistors are utilized extensively in high powerapplications. They are typically deeply diffused devices that carry veryhigh current (1-5 amps or higher). FIG. 1 shows a cross-section of astandard deep diffused PNP device 10. The emitter 14 is the insideelectrode. As shown, this can represent a bulls-eye pattern with theemitter as the bulls-eye and the collector surrounding it, or doublecollector lines adjacent to each side of an emitter. The device 10includes two P+ collectors 12 a and 12 b and P+ emitter 14 between. Thetwo P+ collectors 12 a, 12 b and the P+ emitter 14 are diffused in an Nepitaxial layer 15. An N+ layer 18 (the buried layer) is deposited in aP− substrate 20 which is coupled to the N epitaxial layer 15. An N+ basecontact 16 is coupled to the surface of the N epitaxial layer 15 andcoupled electrically via the N+ buried layer to apply a voltage betweenthe N base and the P+ emitter 14. The collectors 12 a and 12 b include ametal layer 12 a and 12 b respectively on the surface thereof. Thecollectors TCF 12 a and 12 b are outside circular electrodes when abulls-eye pattern is used and are parallel separate structures whenparallel inline design is used. The base 16 is contacted via a metalizedN diffusion that is placed in the N epitaxial layer 15. For someapplications it is tied to the N+ buried layer.

Injection from the emitter 14 is from the total outside periphery of theemitter for the total depth of the emitter. This results in a tremendousdifference in the base widths since the surface portion of the emitter12 is closest to the edge of the surface of the collectors and thereforehas the shortest base width. Moving down the periphery of the emitter 12a and 12 b, the base width becomes longer and longer and reaches itsmaximum base width at the deepest point 23.

Referring back to FIG. 1, it is obvious that most of the injection andcollection could be considered coming from two transistors in parallel.Transistor XR being on the right half of the emitter and its injectionbeing collected by transistor XR. While at the same time transistor YLon the left half of the emitter has its injection being collected by YL,the collector on the left side. To give some general quantitative ideaof the basewidths, assume that the distance from the emitter tocollector is 8 μm on the mask. If, for example, the P diffusions are 2.5μm deep and the side diffusion is 2.0 μm around the total periphery ofthe emitter and collector, then this leaves the basewidth approximately4 μm long at the surface and approximately 8 μm long from point 23. Infact, these effective basewidths are much less due to the depletionregion extending from the collectors into the base region 16 (Nepitaxial layer) and the depletion region of the emitters extending intothe base region 16 (N epitaxial layer). This particular example may notbe able to work at very high voltages due to punch-through. It is veryeasy to have depletion widths of a micron or more. This leaves thesurface with a basewidth of approximately 2 μm and the basewidth of thebottom of approximately 6 μm.

Without surface effects, this means the surface portion of these twotransistors in parallel have the highest beta and the best frequencyresponse, while the deep points have the lowest beta and the worstfrequency response.

In general, the beta coming from the bottom point 23 of thesetransistors can be ignored. Beta is much lower than is achieved at thesurface mainly limited by recombination in the bulk as well as the factthat the base emitter voltage (V_(be)) is somewhat less at the bottomdue to some additional drop from the base contact to the actual base.Likewise, it can be assumed that some of the surface beta is lost due tosurface recombination velocity. It can then be assumed that the beta isthe average beta with a base width of approximately 3 μm. However, betais a function of the amount of current collected versus the amount ofcurrent emitted. The current being emitted is along the total peripheryof the two transistors in parallel as determined by the base emittervoltage (V_(be)) and the resultant low base current. The current beingcollected as a result of this emission is much less due to the issuesjust discussed, therefore resulting in a beta that is much lower.

The frequency response of the standard lateral PNP is determined by theworse response of the structure. This means the bottom of the radialstructure is determining the frequency response of the device due to itslong basewidth. Frequency response is determined by where the output isdown to 0.707 of the low frequency output. At low frequency the current,and therefore the beta is made up of all the varying basewidths from topto bottom of the structure.

As the frequency increases the bottom of the structure with the longbasewidth has recombination occurring on the long basewidths and theoutput current for a given input current goes down. This shows the totalstructure as having a lower output current as the frequency increases.The long basewidth device is therefore determining when the overalloutput is down to 0.707 of the low frequency output. For a power lateralPNP device, where frequency response may not be an issue this is ofsecondary concern; gain is the primary concern.

Another issue with the standard approach relates to debiasing of theemitter when carrying high current. This occurs because metal is on topof the emitter and therefore the maximum voltage is applied on thesurface and the voltage drops to lower values as one goes along thedepth of the emitter due to drops in the resistance of the emitter. Thisdrop can be very high since the current may run 1 amp to greater than 5amps and any amount of resistance will result in significant debiasing.

An ideal lateral PNP would have a profile as shown in FIG. 2, where theemitter and collectors are vertical spikes that have the same basewidthfrom top to bottom. It would also have metal the full depth to reducedebiasing.

Accordingly, what is needed is a system and method for providing a powerlateral PNP that approaches this ideal structure. This lateral PNP wouldhave an improved beta and frequency response. The present inventionaddresses such a need with two approaches.

SUMMARY OF THE INVENTION

A power lateral PNP device is disclosed which includes an epitaxiallayer; a first and second collector region embedded in the epitaxiallayer; an emitter region between the first and second collector regions.Therefore slots are placed in each of the regions. Accordingly, in afirst approach the standard process flow will be followed until reachingthe point where contact openings and metal are to be processed. In thisapproach slots are etched that are preferably 5 μm to 6 μm deep and 5 to6 μm wide. These depths are examples. They can be changed for differentthicknesses of epitaxial material or for different junction depths inthe given technology. These slots are then oxidized either thermally orby deposition of an oxide or dielectric layer and will be subsequentlymetalized. When used for making metal contacts to the buried layer orfor ground the oxide is removed from the bottom of these particularslots by an anisotropic etch. Subsequently when these slots receivemetal they will provide contacts to the buried layer where this isdesired and to the substrate when a ground is desired.

In a second approach the above-identified process is completed upthrough the slot process without processing the lateral PNPs. With aseparate masking and etching, the oxide is removed from the PNP slotsand boron is deposited in a diffusion furnace and driven in anon-oxidizing atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section of a standard deep diffusedPNP/standard IC lateral PNP.

FIG. 2 illustrates an ideal lateral PNP.

FIG. 3 illustrates slotted and oxidized lateral PNP slots.

FIG. 4 illustrates 5 μm metal in 5 um slot-conformal CVD metal—metalfolds in slot and surface metal is only half the thickness.

FIG. 5 illustrates metalized-oxide isolated-slots on PNP.

FIGS. 6A and 6B illustrate standard boron diffused after boron implantfollowed by slot formation and metal deposited in the slots of theemitters and collectors.

FIG. 6C illustrates slots formed first and followed by boron depositionand diffusion (shallow) and then filled with metal.

FIGS. 7A and 7B illustrate metal filled slots where boron was previouslydiffused to form P junctions around the total slots and demonstrateslaminar flow of current from the emitter to the collectors.

FIGS. 8A, 8B, 8C and 8D illustrate slots for various thickness ofepitaxial material where the slots are formed before the P+ emitter andcollector diffusions are done.

FIGS. 9A, 9B and 9C illustrate examples of different thickness (10 um, 5um and 2.5 um thickness, respectively) of epitaxial material where thePNP P+ for emitters and collectors are formed prior to the slot etch.

DETAILED DESCRIPTION

The present invention relates generally to high current semiconductordevices and more particularly to a power lateral PNP device using aburied power buss. However, the technique is advantageous to all uses ofa lateral PNP device since it results in major advantages in die sizereduction, reduction of Ron, improved frequency response and highergain. Two approaches will be discussed as part of this invention. Thereare advantages to both approaches depending on the use of the LateralPNP in the circuit application and the structure of the total devicewhere the lateral PNP is being used.

The discussion will center around the lateral PNP but it is important torealize that the lateral PNP as described is used in BiCMOS, Bipolar,DMOS, LDMOS, CMOS, and any technology where a lateral PNP will be used.The details of the processing for these technologies will not be coveredand is assumed known. So, discussion will jump from starting material,buried layer, epitaxial, and then jump to the lateral PNP which iscontained in these various technologies. The following description ispresented to enable one of ordinary skill in the art to make and use theinvention and is provided in the context of a patent application and itsrequirements. Various modifications to the preferred embodiment and thegeneric principles and features described herein will be readilyapparent to those skilled in the art. Thus, the present invention is notintended to be limited to the embodiment shown but is to be accorded thewidest scope consistent with the principles and features describedherein.

FIG. 2 shows a cross-section of an ideal lateral PNP device 100.

A much higher beta could be achieved if the following existed:

1. The emitter 104 looked more like a vertical stripe facing thecollector as shown in FIG. 2.

2. The collector 106 looked more like a vertical stripe as shown in FIG.2.

3. Surface recombination velocity effects were reduced.

4. The voltage base to emitter (V_(be)), which determines the basecurrent and the bias across the total emitter results in equal biasalong the total emitter depth.

With this type of device the basewidth of the top or the bottom of theemitter structure would be the same. Emission would be along the totalemitter periphery and depth: and, assuming a narrow basewidth fromemitter to collector, collection would be high.

In addition, if a structure were made as shown in FIG. 2, there would bea much higher frequency response since the basewidth would be lessdrastically affected along the whole structure depth and width, andwould more closely follow the basewidth normally obtained at the surfacewhere the basewidth is the narrowest. Heretofore such a device was notpossible utilizing conventional processing technologies.

U.S. patent application Ser. No. 10/034,184, entitled “Buried Power Bussfor High Current, High Power Semiconductor Devices and a Method forProviding the Same,” filed in Dec. 28, 2001, and assigned to theassignee of the present application, describes a method and system forproviding an interconnect on a semiconductor device. The application isincorporated in its entirety herein. The method and system comprisesproviding a semiconductor substrate with a plurality of devicestructures thereon and providing at least one slot in the semiconductorsubstrate. The method and system include providing a metal within the atleast one slot.

This first metal in a preferred embodiment consists of three depositionsof metal when sputtered, with the first two depositions being buried inthe silicon prior to a dielectric and a third deposition of what iscalled the first metal layer. This third deposition provides the normalinterconnect pattern as it normally is patterned in standardmetalization schemes. One therefore has the same metal thickness to etchfor the interconnects but approximately three times this thickness inthe slots.

This interconnect scheme is utilized to advantage to provide a powerlateral PNP device which has improved performance. To describe thesefeatures of lateral PNP device in accordance with the present invention,refer now to the following discussion in conjunction with theaccompanying figures.

Two embodiments for lateral PNP devices are described herein. The firstembodiment describes a lateral PNP device which provides improved betaand frequency response as well as other advantages which will becovered. The second embodiment includes those advantages and alsoreduces the effect of debiasing of the emitters of the device.

First Embodiment Power Lateral PNP using the Buried Power Buss

First the normal processing steps for providing the power lateral PNPdevice are performed. Fabrication of the power lateral PNP would followthe normal processing that is carried on to produce the PNP device. Thisprocess flow will be followed until reaching the point where thecontacts are to be made prior to metal.

At this point, slots are etched that are 5 μm to 6 μm deep and 5-6 μmwide (for this example). These slots are then oxidized. For slots whichare intended to make contact to the buried layer or to the substrate forground, the oxide is removed at the bottom of the slots using ananisotropic dry etch. The slots for power buss retain the oxide.

FIG. 3 illustrates the PNP 200 device after the slots 202 a, 202 b and202 c have been cut simultaneously for the collector, emitter andcollector respectively. A review of FIG. 3 shows that the slots (orcircular tubes in the bulls-eye approach) are made to cut down throughthe emitter in the middle, separating the emitter into two separateemitter structures 205 a and 205 b (physically for the inline structureand theoretically for the bulls-eye structure). This emitter slot 202 bwill have the bottom oxide 208 removed where one desires that theemitter 205 a and 205 b needs to be grounded. In cases where the emitter202 is not to be grounded, the oxide will be left intact. The oxide isremoved wherever it is desired for the P+ to contact the substrate toprovide a ground in the circuit. In this manner grounds can be formedwherever they are desired without requiring the metalization to connectgrounds. This provides an additional alternative for the designer in hislayout since he doesn't need to be concerned about routing the metalfrom P+ to P+ to provide a ground. The slot 202 is made to be almost aswide as the dimension at the surface. In this manner the emitterstructures 205 a and 205 b now look more like vertical spikes withoutmuch curvature since the P+ is from an implanted source and has lessside diffusion than the depth (which is now eliminated).

For the lateral PNP, slots are etched as shown in FIG. 3. The slots areetched down through the emitter leaving only the outside edges of thediffusion remaining. Likewise, slots are etched down through the P+collectors as shown in FIG. 3. This results in only the outsideperiphery remaining on the emitter and only the inside edges of thecollectors (edges facing the emitter). These slots are filled withmetal. This is accomplished by CVD metal deposition which conforms tothe curvature of the slots. As shown in FIG. 4, the thickness of themetal in the slot is twice the thickness on the surface due to thedeposition of the metal “folding” in the slot. This is accomplished byhaving the slot width being twice the dimension of the metal that isdeposited. This provides a major advantage since it lowers the sheetresistance of the metal while allowing the normal thickness to be etchedfor the interconnects. If CVD metal is not available, the metal isapplied in layers as shown in FIG. 5. Layer 1A would be a sputteredmetal 2.5 μm thick. This is followed by a planarization step thatremoves the surface metal and leaves resist in the slots over metal inthe slots. The resist is removed and a second deposition of metal andsubsequent processing results in two thickness of metal in the slots andnone in the field. A dielectric is deposited such as TEOS and the normalcontact openings are then processed, and include the contact etching forcontacting the emitter and collectors of the lateral PNP transistor.This approach results in only the outer edges of the emitter and theinner edges of the collectors remaining and they simulate spikediffusions like the ideal lateral PNP.

The slots 202 a and 202 c for the collectors 210 a and 210 c are on theoutside two thirds of the normal collector as shown in FIG. 3. Thisallows the remaining inside of the collectors 210 a and 210 c to lookmore like vertical spikes since the bottom curvature is removed by theslot and the side curvature is minimal due to the reduced side diffusionof the implanted Boron. The oxide at the bottom of these collectors 210a and 210 c could also be removed or remain intact depending on the typeof device being manufactured and how one wants to use the lateral PNP.

If the oxide is left it will result in the collectors 210 a and 210 cnot contacting anything below the surface. If it is wanted to makecontact to the buried layer, the oxide is removed. If the oxide isremoved the collector could make contact to the epitaxial, the substrateground, or to the N+ buried layer if desired. When it is desired for thecollectors to make these kinds of contacts, a N+ or P+ implant should bedone prior to the oxide removal to ensure a good, non Schottky contactis made.

The base slot is made also in the same manner. In this case the oxidewill be removed from the bottom of the slot allowing the base to makecontact to the N epitaxial or to the N+ buried layer if desired. Inorder to ensure a low contact resistance this slot should have an N+implant prior to removing the oxide. This will result in a lowresistance base contact that is buried as it makes contact to the buriedlayer via the N+ contact and N epitaxial layer.

It should be noted that the discussion to this point assumes the slotsare made at the same time as buried power slots are made. This maycompromise the specifics desired in the Lateral PNP. If this is the casethe slots for the Lateral PNP can be made separately from the buriedpower slots with one additional mask. This would allow the Lateral PNPSlots to be formed differently than the buried power slots to providesome advantages.

The next step in the process would normally be contact opening followedby the conformal deposition of CVD metal. CVD metal is conformal andwould be as shown in FIG. 4. Note that the thickness of metal will bethe slot depth as long as the metal is conformal and is one-half theslot width thickness.

Where CVD metal is not available the metal will be deposited by sputterdeposition. This metalization is done in three simple steps. FIG. 5illustrates the power lateral PNP device after the first, second andthird metals are provided. Immediately after oxidation of the slots andremoval of oxide where needed, a first metal 402 of 2.5 μm is deposited.This is followed by a resist planarization etch to remove the metal onthe surface and then the resist is stripped. This leaves metal of 2.5 μmin the bottom of the slots. Metal 404 of 2.5 μm is then depositedfollowed by a second resist planarization etch and resist strip. Thisleaves 5.0 μm of metal 402 and 404 in the slots and none in the field.This is followed by deposition of TEOS. After this, contacts are openedin all the normal contact opening points as well as above the buriedpower busses that now exist. Another 2.5 μm of metal 406 is depositedand the metal is patterned as normal. This leaves 7.5 μm of metal 406 inthe grounds, power busses, emitters, collectors and wherever the metal406 is needed to reduce ohmic drop as well as to prevent debiasing. Ifit is desired to go out of an epitaxial tub across ground, the groundonly has two layers of metal 402 and 404 with TEOS on top and the metal406 crosses over. Since metal 406 can be routed anywhere that it isneeded without concern for passing over other structures it can betreated like a second metal except for the ground cross-overs.

For this reason metal 406 is referred to as metal one point five (1.5)instead of double metal yet it services just as a double metal processserves to provide advantages. The approach of using metal 406 is carriedout, however, utilizing only one contact opening. The first contactopening is eliminated, whereas the conventional approach for dual metalrequires an extra deposition of dielectric and two contact opening(including the via).

This results in steps such as slots being extra but other steps beingeliminated to provide a double metal advantage. This results in reducedmasks and reduced processing for a given function. Another advantageexists relative to grounding. Since slots can be placed anywhere onedesires a ground and therefore can achieve grounding by using thesubstrate; metal grounds do not have to be routed on the top surface.This makes designing an IC much easier since routing of ground can bedifficult during layout of the design resulting in compromises thatincrease the size of the die.

Since ground contacts are made anywhere it is desired that a ground slotis to be located, there is no need to be concerned about routing groundsince it appears on metals 402 and 404. This provides an easier methodof layout for the circuit designers since they do not have to beconcerned about ground routing. Each ground is made by a metalized slotwith the bottom oxide removed that contacts the substrate. Likewisesinkers, drains, and other power points are directly connected with thefirst two layers of the buried power buss. Since the buried power bussresults in oxide isolation there is no need for isolation mask and thelong isolation diffusion. Likewise there is no need to provide a sinkermask and diffusion since the sinker points are connected with the oxideisolated power buss.

The same is true for the drain of MosFets. If the epitaxial layer is onthe order of 6 μm thick, or less; then one can drop the buried layermask. The buried layer is deposited and diffused via a blanket implant.After epitaxial deposition and the remaining processes, the oxidizedslots go completely through the epitaxial material and blanket buriedlayer and therefore provide oxide isolation from buried layer to buriedlayer, thus providing the free masking of the buried layer. Likewise,the Up Isolation mask can be dropped and its processing.

The buried power buss results in a very efficient heat transfer since attimes the metal is making direct contact to silicon and at other timesthe metal makes contact to silicon via oxide. Heat transfer throughsilicon is 10 times better than through oxide and 200 times better thanthrough air. For this reason there is reason to believe that the heattransfer resulting from utilization of this method is much better thanthe heat transfer provided by the copper damascene method.

Since the metal on the main current carrying busses is 7.5 μm thick and5 μm wide, electromigration is eliminated or at least put off to veryhigh currents. For any design that is already in use, this use of theslots and the added thickness of metal in accordance with the presentinvention will allow at least an order of magnitude of additionalcurrent prior to any electromigration issues.

If it is desired that the current carrying capability be doubled, doubleslots of 5 μm wide are made, with 3 μm of distance between them. Thisresults in metal that is 10 μm wide and 7.5 μm of metal thick. Ifadditional metal layers are to be added to this process, after the metal406, this can be done. This requires an additional planarization stepearlier in the process (after first metal layer). This allows for multimetal structures above the—metal layer described.

As before mentioned, FIG. 5 shows the slots as metalized. In additionFIG. 5 shows the figures with the P sections of the lateral PNP beingformed by a heavy implant and drive, resulting in the lateral diffusionbeing only 0.5 of the diffusion depth. This leaves the emitters andcollectors as shown in FIG. 5 with very little curvature remaining inthe diffusion. In addition, the bottom is cut out of the diffusion bythe slots where the biggest curvature had occurred. This results infairly even distribution of the basewidth going from the top of theemitter and collectors to the bottom. In essence a form of spiked Pdiffusion into the N epitaxial has been achieved. With this structurethere are two separate transistors since the one on the left is oxideisolated from the one on the right. This approach has the followingsignificant advantages over a standard process:

1. Emitters and collectors are spike diffusions resulting in higher betadue to the controlled and equal distance from top to bottom, resultingin improved base width control over the total thickness of the emitterand collectors.

2. The curved bottoms of the diffusions are removed by the power bussslots, therefore eliminating one of the big reasons for inefficiency inthe normal lateral PNP beta, thus improving the beta.

3. The base contact is a buried contact thereby reducing any leakage dueto surface effects. This will result in higher beta due to reduced baseleakage.

4. Using MOS techniques throughout the process to ensure low oxidecharge and low surface recombination velocity results in reduced surfacerecombination and lower leakage issues.

5. Since the base contact may be a slot that goes down to the buriedlayer it results in a non-pinched base region and therefore low rb′.

6. Since the collectors are significantly reduced in area interfacingwith the epitaxial this results in low Cc for a given current and powerlevel in a PNP collector.

7. The emitter capacitance is reduced due to the elimination of most ofthe emitter edge area that has been eliminated.

8. All parameters of the lateral PNP are now improved beyond the normaldevice. There is higher beta due to controlled and equal distance ofbasewidth and truncation of areas that were very inefficient. Theresistance of the emitter and the collector are reduced through directcontact with metal along their total “surface”.

9. The emitter is shown as grounded but in applications like a lowdropout regulator where the emitter is not grounded, the oxide would beleft at the bottom of the slot. In addition, with an extra mask aspecial slot is made and metalized for the emitter that does not haveoxide on the sides of this slot. Therefore the total metal makes contactto the emitter from top to bottom of the emitter and prevents debiasingof the emitter at very high currents. This is an advantage in all highpower lateral PNPs in LDOs and other structures.

10. The buried power buss results in very low resistance in the groundand power busses to the PNP and low power drops.

11. The buried power buss results in high heat transfer. The lateral PNPmade in this manner will have a significantly improved heat transfer perwatt of power dissipated due to metal in slots that are in very closeproximity to the active emitter and collectors.

12. Electromigration is essentially eliminated using the buried powerbuss with this PNP structure.

13. Utilizing this structure in low dropout regulators will result inbeing able to supply a superior product relative to achieving low dropout voltage for a given current and power level.

14. Higher F_(t) and F_(max) for the same given area than any standardapproach used to date.

15. Although the slots are shown for a power lateral PNP the sameapproach can be used for low power lateral PNPs to improve the frequencyresponse and improve beta at a given current level.

Second Embodiment Using Buried Power Buss and Emitter and CollectorSlots Processed Prior to Boron Deposition and Diffusion

Using essentially the same process flow as discussed above, animprovement will be discussed whereby the PNP process is not carried outin the same place to effect an even better structure.

The process flow remains as standard up to the contact opening step,except the processing of the lateral PNP is skipped up to this point.The buried power buss slots are processed and the slots are oxidized. Atthis point there is a major departure.

Where the slots are not completely through the epitaxial material a maskis employed to provide for etching the oxides off the slots that wereformed for the emitters and collectors of the lateral PNP. This isfollowed by a deposition of boron 502 shown in FIG. 6C which will bedeposited along the total inner edge of the emitter and collector slots.This is followed by a diffusion drive which is much shorter and at alower temperature than the standard process and forms Boron junctionsalong the inside periphery of the slots. This diffusion is done in anon-oxidizing process. This is followed by the metal depositions ofmetal 402, 404 and 406 as described in the approach above and the wafersare processed to completion. If a slight oxide is formed in these slotsduring the boron diffusion it should be dipped out with an oxide etch.If the boron is elemental boron it may need to receive a low temperatureoxidation followed by a dip etch to remove the elemental boron and oxideand eliminate any contact resistance. If the application is such thatthe power buss slots go completely through the epitaxial material, theslots for the lateral PNP are processed separately using an extramasking. This is done immediately after the other slots are oxidized.Shallower slots are formed using this extra mask and are not oxidized.Therefore when metal is deposited as discussed it will contact the totalemitter and collector periphery and depth.

An option to the PNP structure is to process the PNP slot so as to leavea dielectric, for example, oxide, in the bottom of the PNP emitter andcollector slots prior to the boron doping. This produces the structuresshown in FIG. 7B where there is no boron dopant diffused from the slotbottoms or metal contact to the slot bottoms. This structure reduces thesubstrate injection from the emitter which enhances the lateral PNPdevice. This process option of selectively leaving a dielectric on theslot bottom can be accomplished through various means, for example: oncethe PNP slots are clear of oxide in the above description, either byoxide etching or slot etching, a thin oxide (100-500A) could bethermally grown or deposited by CVD techniques followed by thedeposition of a silicon nitride layer of thickness 200A to 1000A.Following this with the removal of the silicon nitride on horizontalsurfaces by an anisotropic etch to produce silicon nitride spacers onthe slot walls. These silicon nitride spacers are then used to mask theemitter and collector slot walls from a second thermal oxidation, whichgrows sufficient oxide thickness (1500A-5000A) on the slot bottoms tomask the boron dopant and to prevent metal contact after the removal ofthe side wall nitride and underlining oxide.

These processes result in metal in the buried power buss slots that areoxide isolated from all other elements except where the oxide is removedin the bottom of the slot to contact ground or to contact buried layer.At the same time the metal in the slots just formed for the emitters andcollectors of the lateral PNP is making direct contact with the boron inthese slots since these slots were not oxidized. This results in theideal structure shown in FIG. 6C. This structure has the followingcharacteristics and advantages over any other approach:

1. The emitters and collectors are self-aligned by means of the slotsdetermining their location.

2. The emitters and collectors are spike like structures since they aredetermined by the edge of the slots and the shallow junctions protrudingfrom them. This second embodiment has more ideal spike like diffusionsthan the first embodiment.

3. The basewidths are the same at the top of the structure near thesurface; as well as for the full depth of the slots.

4. The concentration of the emitters and collectors are the same at thetop of the structure as at the bottom of the structure, resulting inequal injection along the total edge of the emitter.

5. Since the junction depth is actually shallow and is only perceived tobe the depth of the slots, injection is determined by a highconcentration rather than the normally lighter doped deep diffusedjunctions at any depth.

6. Since the junction depth is actually shallow and is determined by theslot depth, the slots can be moved much closer to each other than thestandard process. This results in a narrow basewidth from the top of thestructure to the bottom of the structure.

7. Because of the narrow basewidth and high concentration of boron, theemitter efficiency is very high and the transport factor and collectionefficiency is very high resulting in the beta and frequency beinghigher.

8. As a result of the advantages mentioned in 5, 6 and 7 above, thestructure is smaller for a given current level.

9. Debiasing of the emitter is eliminated since metal is thick in theslots and follows the contour of the emitter. The “folding of the metalin the slots reduces the sheet resistance significantly. This results inan equal potential emitter metal bar and current density being equalalong the total emitter structure. Because the metal is making contactalong the total emitter, current is in parallel rather than in series asit is in a structure with the metal contact at the top. This is a majoradvantage of this structure. Current flows in a laminar type conditionfrom the emitters to the collectors as shown in FIG. 7A. This flow ofcurrent results in no debiasing of the emitter at high current, as isthe case in all standard emitter structures at high currents. There isessentially no IR drop in the thickly metalized emitter slots.

10. As a result of the items indicated above, this structure results inhigh beta from low currents to higher currents and from low frequenciesto higher frequencies than other structures. It is as close to an ideallateral PNP as one has achieved to date.

11. An ideal emitter injects from its junction and a diffusion lengthback from this edge toward the metal. This being the case, the depth ofthe boron along the edge of the slot should be made at least onediffusion length deep. This would ensure injection from the total borondepth along the total periphery. To ensure ideal injection without backinjected base current resulting due to interaction with the metal, itmay be necessary to increase the depth of the Boron diffusion to providetwo to four diffusion lengths. This is still a relatively shallowdiffusion compared to the standard approach.

12. This device structure can be achieved without an added mask sincethe normal masking for the lateral PNP would be skipped and replaced asan “emitter/collector” slot mask as detailed previously.

Examples—Boron Diffused Slots

FIGS. 6A and 6B illustrates standard boron diffused after boron implantfollowed by slot formation and metal deposited in the slots of theemitters and collectors. If properly carried out, this process willresult in the outer edges of the emitters and the inner edges of thecollectors boron P+ plus (+) remaining as pseudo spike diffusions. Thisresults in a more uniform injection and collection across a broad rangeof current. Metal in the slots results in much less debiasing than thestandard approach. Beta is higher and flatter over a broader range ofcurrent.

FIG. 6C illustrates slots formed first and followed by boron depositionand diffusion (shallow). This is followed by metal in the slots. Resultsin almost perfect spike type structures for lateral injection andcollection because of the lack of slope to the diffusion. Debiasingeliminated due to metal filling the slots and presenting a lowresistance with current being carried in parallel rather than series asis the standard approach. This approach results in higher beta and betaretention over a broader range of current. Debiasing is eliminatedbecause of the profile of the metal extending along the total length ofthe boron diffusion as well as the thickness of this metal.

FIG. 7A illustrates metal filled slots where boron was previouslydiffused to form P junctions around the total slot's periphery—resultingin laminar current flow because of equal basewidth and thick metal inthe slots with very little voltage drop resulting in an equal potentialslot bar.

If P+ emitters and collectors for the lateral PNP are not put in untilafter the buried power buss slots have been put in and been oxidized,there is a very powerful advantage due to the resulting structure asshown in FIGS. 6C and 7A.

After oxidizing the buried power buss slots, slots would be masked andetched for the emitters and collectors of the lateral PNP. Boron wouldbe deposited into these slots and driven slightly. They do not have tobe driven far since the slot depth determines the depth of theseemitters and collectors and the boron just follows the outline of theslots. After the slight drive of the boron, the metal is deposited inthe buried power buss plus the emitters and collectors (that are notoxidized). Wafers are then processed to completion.

What this now provides is the following:

Metal is in the buried power buss slots and is therefore oxide isolatedfrom other elements. The emitters and collectors of the lateral PNP haveboron along the whole surface of the slots with metal in their slotsjust like the buried power buss metal. The emitters and collectors areself aligned by means of using these slots as receptacles for the boronand metal. Boron is no longer following a diffusion profile, but isfollowing the slot profile. This means boron is quite even along thewhole surface (where surface is defined as the total slot surface) ofthe slots in a sharp profile, as well as the metal that is deposited inthe slots from top to bottom. This means that debiasing is not onlyeliminated, but one achieves equal injection and collection andtherefore beta along the whole profile of the emitters and collectors.The emitters and collectors look like a step function (abrupt junctionwith no curvature). All things being considered it gives an idealinjection and collection lateral PNP with heavy metal along the wholeinjecting and collecting fronts that ensures no debiasing and no dropoff of beta at the normal current presently being experienced. Beta willdrop off at a much higher current level than the present method. Whenthis procedure is properly carried out, a higher beta is provided overthis higher current range.

It is important to have the Boron diffusion depth to be greater than thecarrier diffusion length away from the metal. This is to ensure properinjection without interference by the metal. Since metal is in themiddle of the slots, injection will occur from the total diffusion depthon the edges of the slots. This may be achieved without an extra mask,since the mask used for defining the deep P type emitters and collectorsis not used in the normal flow and is used for these “active” lateralPNP slots.

Examples—Various Epitaxial Thicknesses

One of the advantages of utilizing a system and method in accordancewith the present invention is that the epitaxial layer can be of variousthicknesses as appropriate. To describe this feature in more detail,refer now to the following discussion in conjunction with theaccompanying figures.

FIGS. 8A, 8B, 8C, and 8D illustrate slots for various thicknesses ofepitaxial material. FIG. 8A illustrates 10 μm thick epitaxial material Ntype. The slots are 6 μm deep and whatever width is required for thegiven process. In this approach the slots are etched prior to the P+doping of the lateral PNP emitters and collectors. If it is desired thatthe emitter be grounded there is a P+ buried layer that diffuses up andmakes contact with the P+that is on the edge of the slots. If theemitter is not to be grounded there is no buried P+. However, there maybe occasion to have the collectors grounded or to form a ground at anyremote point in the circuit. This can be accomplished using the approachjust mentioned.

FIGS. 8B and 8C illustrate a 2.5 μm thick N epitaxial material. FIG. 8Billustrates 6 μm slots and has an N+ buried layer to keep the P+ thatgoes to the substrate from shorting the emitters to collectors. FIG. 8Cillustrates a 2.5 μm epitaxial and the slots are only 1.5 μm deep. Thiswill result in the slots being filled with one deposition of metal 1Aand does not require a second deposition 1B. The device continues to usemetal deposition 1C as the interconnect metal as well as contacting allof the slots. The total metal at the slot positions will beapproximately 3.5 μm thick and 1 μm thick in the interconnects as anexample.

FIG. 8D illustrates a 1 μm thick epitaxial material with slots that areone half micron deep. If the slots are two microns wide and one micronof metal is deposited (for example), it will result in metal 1.5 μmthick in the slots due to the folding of the metal and 1 μm thick in theinterconnects. As a result, the interconnects are easy to etch andinclude 1.5 μm metal in the slots for carrying high current.

FIGS. 9A, 9B and 9C illustrate examples of different thicknesses (10 μm,5 μm and 2.5 μm thickness, respectively) of epitaxial material where thePNP P+ for emitters and collectors are formed prior to the slot etch.The slot cuts through the emitter and leaves the outer edges to face thecollectors. The slot cuts through the collectors in a way to only leavethe inner edge of the collectors with P+ showing. The slot cuts looklike P+ spikes of collectors facing P+ spikes of emitter on the outeredges of the emitter. This approach uses the standard process flow andputs the slots in when doing the buried power buss slots. This approachdoes not give the sharpness of “spike” P+ emitters and collectors as theapproach where the slots are put in first and the P+ diffusions done forthe emitter and collectors.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A system for providing a lateral PNP device comprising: means forgrowing an epitaxial layer; means for providing first and secondregions, the first and second regions becoming first and secondcollectors after a diffusion; means for providing a third region betweenthe first and second regions, the third region becoming an emitterregion after the diffusion; means for etching slots in each of theemitter and first and second collectors; means for doping each of theslots with boron; and means for filling each of the doped slots withmetal.
 2. The system of claim 1 wherein the slot in the emitter regionprovides for separate emitter structures.
 3. The system of claim 1wherein the metal filling means is performed by CVD metal deposition. 4.The system of claim 1 wherein the metal filling means is performed byapplying the metal in layers and planarizing the metal.
 5. The system ofclaim 1 wherein the slot in the emitter region is not oxidized at thebottom.